A method and arrangement of this type is for example disclosed in U.S. Pat. No. 6,024,449, wherein a grid pattern of pulsed monochromatic polarized light is projected onto a semi-difuse target surface, such as the de-epithelialized cornea of an eye undergoing photo refractive keratometry (PRK) or photo therapeutic keratometry (PTK), for performing high speed topography measurements on said target. In addition, the method and arrangement disclosed include respectively enable measurement of regions of overtemperature (hot spots) on the target surface using a quantum well infrared photodetector.
Many industrial, scientific and medical processes involve the measurement of the topography of surfaces for variety of applications. In most cases the accuracy of the measurements is of great importance for the quality of the output of the process mentioned. A specific type of surface topography measurements involves the measurements of curved surfaces, as is applied, for example, in ophthalmology where the topography of the corneal surface of the eye provides an indication of the quality of the eye and possible deflections from a healthy eye.
Some methods of measuring the topography of surfaces are based on specular reflection, e.g. a wave-front of a collimated coherent beam of light is projected on the surface and its reflected part is compared with an undisturbed reference beam (interferometric measurement) or a pattern is projected or mirrored by the surface and its reflection is compared with a reference pattern.
The limitations of these methods are that the accuracy is greatly reduced when both convex and concave curvatures are present on the surface or when rays are reflected outside the aperture of the sensor.
A solution is the application of moiré methods. Moiré methods are a versatile set of techniques for in-plane and out-of-plane deformation measurements, topographic contouring, and slope and curvature measurement. The basis of moiré methods are grids, which for use in eye surface topography may enable the projection of a pattern of lines onto the surface. Detection of the line pattern on the surface and overlaying the line pattern by an undisturbed reference pattern visualises the moiré pattern, which comprises the required information about the topography of the surface.
For detection of the projected pattern, a diffusely radiating surface is desired for this technique. For specular reflecting surfaces like the eye, in prior art, these surfaces are transformed in a diffusely radiating surface through the application of fluorescein on the surface. A diffuse reflection of the projected pattern can then be detected by the detection means.
A disadvantage of this technique is that it is marginally invasive. Especially for ophthalmologic purposes this is not desired since the application of fluorescein onto the eye disturbs the tear film and makes it difficult to see references, such as the pupil or the iris, on or beneath the surface of the eye. Another drawback of this technique is that it does not work on dry surfaces, such as dry eyes.
A good example of a method and arrangement used for eye topography, wherein a derivative of moiré methods and the application of fluorescein is used, is described in European patent EP 0 551 955. This document describes some features of such a kind of imaging in surface topography measurements, especially in relation to eye and corneal surface topography, in more detail.
International patent application no. WO 02/45578 is directed to a method for determining the topography of a surface of a biological tissue, comprising the projection of an image onto this surface. The projection is performed with either ultra-violet (UV) or infra-red (IR) light. The method comprises measurement of scattered radiation from the surface as a result of said projection. The method also proposes the use of a thin fluorescent layer for increasing the yield of light to be measured. If UV light is used for determining the topography of the surface, the method may be based on autofluorescence of the surface.
As mentioned above, the use of a fluorescent layer may not always be desired, especially not in application directed to determining the topography of a corneal surface as the fluorescein disturbs the tear film on said surface. Disadvantages of the use of UV light, especially in the range for which autofluorescence occurs, is that at these frequencies a surface such as an eye surface may be damaged due to protein denaturation, which may cause actinic conjunctivitis. If neither a fluorescent film nor UV light is used, but instead the method is only based on analysis of scattered radiation from the surface alone using IR light, the yield of light to be analysed may be very small, since the particles that are to scatter the radiation back, are for a large part smaller than the wavelength used. A person skilled in the art may appreciate that such a scattering process will be inefficient.